Non-passivating barrier layer electrodes

ABSTRACT

An electrode is provided which resists passivation over long periods of time and hence which is especially well suited for use as an oxygen anode. The electrode comprises 1) an electrically conductive supporting substrate, 2) an intermediate, electrically conductive, barrier layer and 3) an electrocatalytically active, solid solution-type, outer coating. The barrier layer is selected from the group consisting of platinum-iridium alloys and oxides of cobalt, manganese, palladium, lead and platinum.

United States Patent [191 Bennett et al.

[ Nov. 27, 1973 NON-PASSIVATING BARRIER LAYER ELECTRODES [63]Continuation-impart of Ser. No. 22,021, March 23,

1970, abandoned.

[52] US. Cl. 204/290 F [51] Int. Cl B0lk 3/06 [58] Field of Search204/290 F [56] References Cited UNITED STATES PATENTS 3,103,484 9/1963Messner 204/290 F 3,663,414 5/1972 Martinsons 204/290 F 3,469,074 9/1969Cotton et al. 204/290 F 3,632,498 l/1972 Beer 204/290 F 3,657,102 4/1972Keith et al 204/290 F 3,654,121 4/1972 Keith et al 204/290 F 3,562,0082/1971 Martinsons 204/290 F 3,616,302 10/1971 Osawa et al. 204/290 F3,632,497 l/l972 LeDuc 204/290 F OTHER PUBLICATIONS Handbook of Chem &Physics, 1963, Chem. Rubber Pub. Co., Cleveland, Ohio, page 425.

Primary Examiner-F. C. Edmundson Attorney-Nelson Littell, Walter G.l-leissenberger, Edward R. Freedman, Nelson Littell, Jr. and Charles A.Muserlian [5 7] ABSTRACT An electrode is provided which resistspassivation over long periods of time and hence which is especially wellsuited for use as an oxygen anode. The electrode comprises I) anelectrically conductive supporting substrate, 2) an intermediate,electrically conductive, barrier layer and 3) an electrocatalyticallyactive, solid solution-type, outer coating. The barrier layer isselected from the group consisting of platinum-iridium alloys and oxidesof cobalt, manganese, palladium, lead and platinum.

7 Claims, No Drawings NON-PASSIVATING BARRIER LAYER ELECTRODES REFERENCETO A CO-PENDING APPLICATION BACKGROUND OF THE INVENTION In the area ofelectrochemical reactions, those processes which employ electrodesfunctioning as oxygen anodes are of considerable commercialsignificance. Examples of some such processes include electrowinning,for example the aqueous electrowinning of antimony, cadmium, chromium,cobalt, copper, gallium, indium, manganese, thallium and zinc; waterelectrolysis metal plating and others. A variety of materials have beenused for the fabrication of such anodes including graphite, platinum,platinized titanium, nickel, lead and lead alloys. These anodes areknown to have various disadvantages which limit their application suchas chemical reactivity, lack of dimensional stability, material cost,contamination of the product, sensitivity to impurities and others.While all of these problems are serious, the overriding disadvantage inall instances is the high oxygen overvoltage exhibited by the anodes.overvoltage refers to the excess electrical potential over theoreticalat which the desired element is discharged at the electrode surface.

Similar problems have plagued the chlor-alkali industry in its attemptsto obtain a low chlorine overvoltage, dimensionally stable anode for usein the production of chlorine and caustic. Thus, electrode developmentin this industry may be traced through the use of graphite andplatinized titanium to the development of a recent composite typeelectrode which appears to be especially well suited for use in anodicapplications. These electrodes consist of a valve metal substrate and acoating'which has variously been characterized as mixed oxide, mixedcrystal, solid solution and ceramic semi-conductor, but which ischaracterized for the most part by being based upon a co-deposit of avalve metal oxide and a non-valve metal oxide. Those codeposits of theforegoing type which have been found to be successful to date haveinvariably been based upon the presence of the oxides in the crystallineform and for the most part in such a manner that an atom of, for examplevalve metal in the valve metal oxide crystal lattice is substituted withan atom of a non-valve metal. Such coatings hereinafter will be referredto as solid solutions. Electrodes of this type have found almostimmediate accpetance as chlorine anodes by reason of their excellentwear characteristics and extremely low chlorine overvoltages.

In view of the similarity in some respects between the problems relatingto chlorine anodes and oxygen anodes, investigators have been led toattempt to employ the successful solid solution-type electrodes asoxygen anodes. In doing so it was observed that the oxygen overvoltagesof some of the more widely used materials are as follow; lead 0.85 volt,platinized titanium 0.62 volt, graphite 0.40 volt, nickel 0.37 volt andsolid solution (ruthenium oxide-titanium oxide on titanium metal) 0.29volt. These overvoltages are measured at 2 amperes per square inch in a1N NaOl-ll solution at 80C.

Thus it can readily be seen that the solid solutiontype coating offerssubstantial advantage in terms of oxygen over-voltage which makes itsuse as an oxygen anode extremely appealing economically. However, it wassoon found that such advantage could not be exploited for anotherreason. Whereas, when the solid solution-type electrode is employed asan anode for chlorine production, the chlorine overvoltage remainssubstantially constant for long periods of time in use, when the sameelectrode is employed as an oxygen anode, the oxygen overvoltage, whileinitially low, steadily increases until, if carried to an extreme, theanode passivates completely, i.e., fails to pass any electrical current.For example, when employed as the anode in sulfuric acid, the anode willpassivate to such an extent as to render further operation uneconomicwithin approximately 30 hours at a current density of 1.0 ampere persquare inch of anode surface area. Therefore, if the advantage of theextremely low oxygen overvoltage available with the solid solution-typeelectrodes is to be obtained, a method of preventing the passivation ofthis coating during use in the evolution of oxygen must be provided.Once it was found that apparently the method of passivation involves theslow diffusion of oxygen through the solid solution-type coating intothe supporting valve metal, attention was directed to some method ofpreventing such diffusion.

STATEMENT OF THE INVENTION Therefore, it is an object of the presentinvention to provide a composite electrode and a method of producingsame which electrode will exhibit improved resistance to passivation,particularly when employed as an oxygen anode.

A further object of the invention is to provide an improved electrolyticprocess involving the generation of oxygen at the anode, which processemploys a passivation-resistant anode.

These and further objects of the present invention will become apparentto those skilled in the art from the specification and claims whichfollow.

It has now been found that the tendency for a solid solution-typeelectrode to passivate when oxygen is generated at its surface, may besubstantially reduced, if not completely eliminated, by the provision ofa barrier layer between the supporting substrate and the solid solutioncoating. Specifically, it has now been found that a surprisinglyeffective electrode comprises 1) an electrically conductive supportingsubstrate; 2) a relatively thin, intermediate, electrically conductive,relatively oxygen-impermeable, barrier layer consisting essentially of amaterial selected from the group consisting of platinum-iridium alloysand oxides of cobalt, manganese, palladium, lead and platinum and, 3) anelectrically-conductive, electro-catalytically active,electrolyte-resistant outer coating consisting essentially of a solidsolution of a valve metal oxide with at least one non-valve metal oxide.Such an electrode not only exhibits an extremely low initial oxygenovervoltage, but retains that low overvoltage through extended periodsof use. Furthermore, the wear-rate, that is, the physical loss ofcoating per unit time, is extremely low.

A further and unexpected advantage of the present invention may also bementioned. While the various patents describe the provision of the solidsolution-type coatings on a wide variety of substrates, it hasheretofore been extremely difficult to apply a true solid solution-typecoating to anything but a valve metal, especially titanium, substrate.When, for example, one attempts to apply a ruthenium oxide-titaniumoxide solid solution onto a steel substrate, a non-adherent, apparentlyamorphous, physical mixture of oxides is obtained which has little or nopractical value as an electrode coating. In the chlorine industry thishas been of little importance for the reason that titanium is in anyevent the preferred substrate material owing to its ability to healitself if exposed to the corrosive cell environment unprotected by thesolid solution coating. However it is obvious that for other, lessdemanding, applications, a less costly substrate, such as steel orgraphite, would in many instances be desirable. It has now been foundthat a barrier layer of the type described apparently exhibits some sortof catalytic activity which assures that the mixture of materialssubsequently applied will form a true solid solution, regardless of thesubstrate.

DESCRIPTION OF THE PREFERRED EMBODIMENTS The invention is definedbroadly as relating to an electrode. The word electrode" is intended torefer to either anodes or cathodes as it will be apparent that thecomposite electrodes of the present invention will carry current ineither capacity. Of course, since the primary advantage of the electrodeis its resistance to passivation, which passivation usually occurs whenoxygen is generated at or near the electrode surface, most applicationsof the electrode will be as an anode, especially as an oxygen anode. Itwill be understood, however, that if, as is suspected at this time, themethod of failure of a chlorine anode also relates to oxygenpassivation, the composite electrodes of this invention will also act aschlorine anodes of extended life.

It should further be understood that the invention is independent of themechanical configuration of the substrate and hence may take any shapewhich will allow the application of the intermediate and other coatingsby the techniques generally described hereinbelow. Thus the electrodesmay take the form of a wire, rod, cylinder, sheet and the like. Further,if the electrode is present in a sheet or plate form, it may be eithersolid or forarninous. Other configurations most useful in a particularapplication will be apparent to those skilled in the art.

The first element of the composite electrode is the electricallyconductive supporting substrate. As indicated above, the identity of thesubstrate is not as limited as it had been heretofore when a solidsolutiontype coating was contemplated. While valve metal substrates,particularly titanium, will still be preferred for many applicationsbecause of their ability to heal themselves under corrosive cellconditions should a defeet in the coating arise, it is now possible touse most any material which has the desired combination of electricalconductivity and mechanical strength. Therefore graphite, steel, copperand the like are also quite useful for numerous applications of thepresent invention.

The intermediate barrier layer has been said to be selected from thegroup consisting of platinum-iridium alloys and oxides of cobalt,manganese, palladium, lead and platinum. These materials may be appliedin relatively thin layers, that is, as low as 0.1 micron, to form anelectrically-conductive layer which appears to prevent the diffusion ofoxygen through the relatively porous outer coating to the underlyingsubstrate. On this basis, any barrier layer meeting the other criteriaherein, especially electroconductivity, which is less permeable tooxygen diffusion than the covering solid solution-type layer, willtheoretically result in improved resistance to passivation. Cobalt,manganese, lead and palladium oxides may be provided directly, and inthe proper crystalline form, on the electrically conductive substrate byelectrolytic deposition using techniques well-known to those skilled inthe art. Platinum oxide is not deposited directly but rather a metalliccoating of platinum is first electrodeposited, followed by a brief heattreatment which appears to convert a substantial portion of the platinumto the oxide form. It appears at this time that the heat treatment iscritical since it has been found that the solid solution coating willnot adhere to the untreated metal itself and other methods of formingthe platinum oxide have proven unsuccessful for the same reason. Theefi'ect of the heat treatment in converting the platinum to the properform may be evidenced by the distinct color change from the originalmetallic finish. The platinum-iridium alloys appear to remainsubstantially in the metallic form even following thermal application ofthe solid solution. These alloys generally, but not necessarily, contain20-50% iridium, typically 30%, and are applied by any of the knownmethods. such as thermochemical deposition from mixed salt solution,which result in an adherent, relatively non-porous layer.

It is the solid solution-type outer coating which gives the compositeelectrode its ability to catalyze a number of electrochemical reactionsat remarkably low overvoltages. This coating consists of a solidsolution of a valve metal oxide with at least one non-valve metal oxide.The term valve metal in this context has its usual significance, thatis, it relates to metals such as titanium, tantalum, zirconium, niobiumand the like. The non-valve metal oxide is chosen to be such that thedesired solid solution is formed, that is, it must have the propercrystal size to mesh with the crystal lattice of the valve metal oxide,usually be substitution of one atom of non-valve metal for one atom ofvalve metal, thus providing electrical conductivity in a normalnonconductive material. Furthermore, it must provide the desiredelectrocatalytic activity. Especially suitable non-valve metal oxidesuseful in the practice of the present invention including platinum,palladium, iridium, ruthenium, rhodium, osmium, molybdenum, tin,tungsten, vanadium, chromium, rhenium, manganese and the like.

In order that those skilled in the art may more readily understand thepresent invention and certain preferred methods by which it may becarried into effect, the following specific examples are afforded.

EXAMPLE 1 A piece of 0.16 inch thick solid titanium sheet (A.S.T.M. B265 581 Grade 2) is degreased with acetone and etched 10 minutes at C.in 20% l-lCl. This sheet is made the cathode in a 2% solution ofchloroplatinic acid in 2.0 N HCl. Platinum is deposited for 10 minutesat room temperature and a current density of 6.2 amperes per square foot(a.s.f.). The electrodeposited platinum metal-coated titanium substratethus obtained is then heated in air for 7 minutes at 450C. A solutionconsisting of 1 gram RuCl -XH O (0.4 gram Ru metal), 6.2 ml. n-butylalcohol, 3.0 ml.

tetrabutyl orthotitanate, and 0.4 ml. 36% HCl is then painted ontothesample surface, andthe sample is heated in air at 450C. for 7 minutes.This painting and heating cycle is repeated five more times to bring thetotal number of coats to six. This electrode when employed as theanodeis a l N NaOH solution at 80C. and an applied current density of2.0 amperes per square inch (a.s.i.), exhibits an oxygen overvoltage of0.29 volts.

When the electrode prepared above is operated as an anode in a 100 gramsper liter aqueous solution of sulfuric acid at 20C. and a currentdensity of 4 a.s.i., it continues to generate oxygen for 110 hours. Anelectrode preparedin the same manner but without the intermediateheat-treated platinum barrier layer, passivates in about 1 hour. Thus itwill be seen that the barrier layer very significantly extends theuseful life of a solid solution-type electrode. Furthermore it should berealized that the life of the anode at 4 a.s.i. is equivalent to severalmonths of operation at normal commercial current densities of from.30-40 a.s.f.

EXAMPLE 2 A piece of 0.060 inch expanded titanium mesh is pretreated asin Example 1 and made the anode in a solution containing 291 grams Co(NO'6H O. Cobalt oxide is deposited for minutes at a temperature of 60C;and a current density of 4.8 a.s.f. Six coats of the ruthenium-titaniumsolution are applied as in Example 1. Oxygen overvoltage is againmeasured at 0.29 volts and 47 hours is required for the anode topassivate (H 80 at 3 a.s.i.

EXAMPLE 3 EXAMPLE 4 A piece of .060 expanded titanium is pretreated asin Example 1 and made the anode in a solution containing 3.3 grams perliter palladium nitrate. Palladium oxide is deposited for 1 hour at atemperature of 50C. and a current density of 2.4 a.s.f. Afterapplication of the solid solution-type coating as before, an electrodeis obtained which exhibits an oxygen over-voltage of 0.29 and a life of37 hours at 3 a.s.i.

EXAMPLE 5 A piece of unimpregnated Union Carbide graphite Grade YAV isground down to expose a fresh surface. MnO is then deposited on thissurface as in Example 3, and six coats of the solid solution coating areapplied as in Example 1. This sample again gave the low oxygenovervoltage of 0.29. The anode is operated for 16 hours without change,it being apparent that passivation will not occur absent thefilm-forming metal substrate.

' EXAMPLE 6 A piece of .016 inch titanium sheet is pretreated as inExample 1 and made the anode in a solution containing 300 grams perliter Pb(NO 2 grams per liter Cu(NO -H O, and 1 gram per liter non-ionicwetting agent. An undetermined amount of lead dioxide is deposited. Sixcoats of the titanium-ruthenium solution are applied as before, with theexception that the bake temperature is reduced to 300C. owing to thelow'decomposition temperature of lead dioxide. X-ray analysisestablishes the presence of the usual solid solution structure and a 12hour test demonstrates that no passivation occurs.

EXAMPLE 7 A titanium metal sheet is provided, by thermochemicaldeposition, with a 70% platinum iridium alloy layer amounting to 4.5milligrams per square inch. Six' applications of the solid solutioncoating are then made as in Example 1. The resultant electrode continuesto operate for 305 hours at 4 a.s.i. in 100 g/l. H2804.

While the invention has been described with reference to certainspecific embodiments thereof, these examples are intended to beillustrative only and the invention should not be so limited sincechanges may be made therein which are still within the intended scope vof the appended claims.

We claim:

1. An electrode comprising:

a. an electrically conductive supporting substrate;

b. a relatively thin, intermediate, electrically conductive, relativelyoxygen-impermeable, barrier layer consisting essentially of one oxide ofthe group consisting of oxides of cobalt and lead and c. anelectrically-conductive, electrocatalytically active,electrolyte-resistant, solid solution-type outer coating consisting ofat least one valve metal oxide and at least one oxide of metal selectedfrom the group consisting of platinum, palladium, iridium, ruthenium,rhodium, osmium, molybdenum, tin, tungsten, vanadium, chromium, rhenium,and manganese.

2. An electrode as in claim 1 wherein the supporting substrate istitanium.

3. An electrode as in claim 1 wherein the outer coating is a solidsolution of a valve metal oxide and a nonvalve metal oxide selected fromthe group consisting of platinum, palladium, iridium, rhodium andruthenium.

4. An electrode of claim 3 wherein the outer coating is a solid solutionof titanium dioxide and ruthenium oxide.

5. In a process for the production of a composite electrode comprisingan electrically conductive supporting substrate and an electricallyconductive, electro-catalytically active solid solution-type outercoating consisting of at least one valve metal oxide and at least oneoxide of a metal selected from the group consisting of platinum,palladium, iridium, ruthenium, rhodium, osmium, molybdenum, tin,tungsten, vanadium, chromium, rhenium and manganese, the improvementwhich comprises providing said electrode with a relatively thin,intermediate, electrically-conductive, relatively oxygen-impermeable,barrier layer selected from the group consisting of oxides of cobalt andlead.

6. A method as in claim 5 wherein the outer coating is provided by thethermochemical co-deposition and decomposition of a mixture comprising avalve metal consisting of oxides of cobalt and lead and anelectrically-conductive, electrocatalytically active, electrolyteresistant, solid solution-type outer coating consisting of at least onevalve metal oxide and at least one oxide of a metal selected from thegroup consisting of platinum, palladium, iridium, ruthenium, rhodium,osmium, molybdenum, tin. tungsten, vanadium, chromium, rhenium andmanganese.

Patent No. D d November 27,

fi fl I John E. Bennett et a1.

It is certified that error appears in the above-identified patent andthat said Letters Patent are hereby corrected as shown below:

On the cover sheet, column 1, after line 6, insert Assignee ELECTRONORCORPORATION, Panama City, Panama On the cover sheet, column 2 line 8"Heissenbrger" should read Neissenberger Signed and sealed this 18th dayof June 197b,.

(SEAL) Attest: v

EDWARD M.FLETCHER,JR. C. MARSHALL DANN Attesting Officer Commissioner ofPatents F Poqoso uscoMM-Dc 60376-P69 I U5 GOVERNMENT PRINTING OFFICEI909 0-866-33L

2. An electrode as in claim 1 wherein the supporting substrate istitanium.
 3. An electrode as in claim 1 wherein the outer coating is asolid solution of a valve metal oxide and a non-valve metal oxideselected from the group consisting of platinum, palladium, iridium,rhodium and ruthenium.
 4. An electrode of claim 3 wherein the outercoating is a solid solution of titanium dioxide and ruthenium oxide. 5.In a process for the production of a composite electrode comprising anelectrically conductive supporting substrate and an electricallyconductive, electro-catalytically active solid solution-type outercoating consisting of at least one valve metal oxide and at least oneoxide of a metal selected from the group consisting of platinum,palladium, iridium, ruthenium, rhodium, osmium, molybdenum, tin,tungsten, vanadium, chromium, rhenium and manganese, the improvementwhich comprises providing said electrode with a relatively thin,intermediate, electrically-conductive, relatively oxygen-impermeable,barrier layer selected from the group consisting of oxides of cobalt andlead.
 6. A method as in claim 5 wherein the outer coating is provided bythe thermochemical co-deposition and decomposition of a mixturecomprising a valve metal oxide and at least one platinum group metaloxide, thereby obtaining a solid solution-type coating.
 7. In a methodof conducting an electrochemical reaction wherein oxygen is liberated atthe anode, the improvement which comprises using as said anode acomposite electrode comprising: a. an electrically conductive supportingsubstrate, b. a relatively thin, intermediate, electrically conductive,relatively oxygen-impermeable, barrier layer consisting essentially ofone oxide of the group consisting of oxides of cobalt and lead and c. anelectrically-conductive, electrocatalytically active, electrolyteresistant, solid solution-type outer coating consisting of at least onevalve metal oxide and at least one oxide of a metal selected from thegroup consisting of platinum, palladium, iridium, ruthenium, rhodium,osmium, molybdenum, tin, tungsten, vanadium, chromium, rhenium andmanganese.